Nowadays, many moving platforms (e.g. aircrafts, ships, cars, etc.) are required to have satellite communication capabilities. One exemplary requirement relates to an entertainment system for offering passengers with e.g. internet access, live television broadcast and the like.
During motion, the moving platform (e.g. the aircraft) is engaged in communication with a particular satellite, tracking it across the sky until it disappears over the horizon, and prior to its disappearance establishes communication with another satellite. Therefore, antennas on-board the moving platforms are typically equipped with suitable positioning and tracking systems.
U.S. Pat. No. 5,796,370 discloses a dual polarization antenna for direct broadcast satellites. The antenna is orientable, directional and capable of use as a transmit and/or receive antenna. It includes at least one reflector, at least one source of electromagnetic radiation including means for exciting the source with two orthogonal linear polarizations and a mechanical system for positioning and holding the source and the reflector. The orientation of the antenna is made up of depointing and rotation about a preferred direction of propagation of the radiation and the mechanical system enables such rotation while keeping the source fixed, so conserving the orientation of the orthogonal linear polarization. A preferred embodiment of the antenna includes a parabolic main reflector and a hyperbolic auxiliary reflector in a Cassegrain geometry, and the mechanical system enables rotation of both reflectors about the preferred direction of radiation and holds the source fixed to conserve the orthogonal linear polarization axes of the beam. Applications include radar, direct broadcast satellites and telecommunications employing frequency re-use by polarization diversity, especially advantageous in space and airborne applications.
U.S. Pat. No. 6,034,634 discloses an inexpensive high gain antenna for use on terminals communicating with low earth orbit (LEO) satellites which include an elevation table mounted for accurate movement about a transverse axis on an azimuth turntable mounted for rotational movement about a central axis. A plurality of antenna elements forming a phased array antenna is mounted on the top of the elevation table and have a scan plane which is parallel to and extends through the transverse axis of the elevation table. The antenna may be both mechanically and electrically scanned and is used to perform handoffs from one LEO satellite to another by positioning the elevation table of the antenna with its bore sight in a direction intermediate the two satellites and with the scan plane of the antenna passing through both satellites. At the moment of handoff, the antenna beam is electronically scanned from one satellite to another without any loss in data communication during the process.
U.S. Pat. No. 6,034,643 discloses a directional beam antenna device that includes an antenna supporting member which is supported on a base in such a manner as to be rotatable about a first rotational axis; an antenna portion which is supported on the antenna supporting member in such a manner as to be rotatable about a second rotational axis which is perpendicular to an antenna aperture and is inclined at a first angle with respect to the first rotational axis, the direction of an antenna beam being inclined at a second angle with respect to the second rotational axis; a first driving unit for rotating the antenna supporting member about the first rotational axis with respect to the base; and a second driving unit for rotating the antenna portion about the second rotational axis with respect to the antenna supporting member. A directional beam controlling apparatus is provided with a controlling unit for controlling an elevation angle of the antenna beam to a target value by causing the second driving unit to rotate the antenna portion with respect to the antenna supporting member, and for controlling an azimuth angle of the antenna beam to a target value by causing the first driving unit to rotate the antenna supporting member with respect to the base.
PCT Application No. WO2004/075339 discloses a low profile receiving and/or transmitting antenna that includes an array of antenna elements that collect and focuses millimeter wave or other radiation. The antenna elements are physically configured so that radiation at a tuning wavelength impinging on the antenna at a particular angle of incidence is collected by the elements and focused in-phase. Two or more mechanical rotators may be disposed to alter the angle of incidence of incoming or outgoing radiation to match the particular angle of incidence.
Also relating to positioning of satellite communication antennas on-board moving platforms are U.S. Pat. Nos. 6,400,315, 6,218,999, 6,741,841, 6,356,239, and 6,751,801.
As is known, polarization of a linear polarized radio wave may be rotated as the signal passes through any anomalies (such as Faraday rotation) in the ionosphere. Furthermore, due to the position of the Earth with respect to the satellite, geometric differences may vary due to relative movements between the satellite and the communicating station (e.g. aircraft, fixed station. etc.). Therefore, most geostationary satellites operate with circular polarization, as circular polarization will keep the signal constant regardless of the above-mentioned anomalies. However, some geostationary satellites use linear polarization. In linear polarization, a misalignment of polarization of 45 degrees will degrade the signal up to 3 dB and if misaligned 90 degrees, the attenuation can be 20 dB or more. Furthermore, polarization purity is required by international regulation of satellite communication. Therefore, on-board antenna systems for communication with a satellite using linear polarization need to provide polarization tracking.
Furthermore, on-board antenna systems for moving platforms are required to be relatively small in size and low in profile (diameter and height) in order to adapt to the overall design and specifically the aerodynamic design of the moving platform. However, polarization tracking typically requires a considerable antenna size, for compensating for losses of signal strength involved in polarization tracking.
There is a need in the art for an improved antenna that provides positioning capabilities as well as polarization tracking capabilities. There is a further need in the art for an improved antenna suitable for use on board moving platforms and specifically airborne platforms and aircrafts, which is relatively small and has low profile (e.g. diameter of about 90 cm or less).